FIG. 1 illustrates a configuration of a wavelength Division Multiplexing (WDM) system.
A wavelength division multiplexing device 11 on the transmitter side includes a plurality of transponders 12a through 12n for converting various types of client signals (such as signals based on SONET, GbE, 10BbE or the like) into optical signals of plural wavelengths, a multiplexer 13 for coupling plural optical signals of different wavelengths, and a WDMPost amplifier 14.
An optical signal obtained by the multiplexing performed by the wavelength division multiplexing device 11 on the transmitter side is transmitted, through an optical transmission line 15, to a wavelength division multiplexing device 16 on the receiver side. The wavelength division multiplexing device 16 on the receiver side includes a WDMPre amplifier 17, a demultiplexer 18 for demultiplexing wavelength-multiplexed optical signals, and a plurality of transponders 19a through 19n for converting optical signals of respective wavelengths into the original client signals.
The transmission speed of wavelength division multiplexing systems has been increased in order to increase their transmission capacity, and as a result of this, optical signals on transmission lines are influenced by dispersion at an ignorable level. This has made it necessary to perform dispersion compensation not only for all wavelengths en masse but also for each wavelength.
FIG. 2 illustrates a circuit diagram of a receiver unit 21 of a transponder having a variable dispersion compensator for performing dispersion compensation for each wavelength.
The receiver unit 21 of the transponder includes an optical amplifier 22 for amplifying optical signals, a variable dispersion compensator 23 for compensating for dispersion of signals output from the optical amplifier 22, an optical-electrical (O/E) conversion unit 24, a signal processing unit 25 for converging electric signals into client signals, and a control unit 26.
The control unit 26 has a function of adjusting the amount of compensation performed by the variable dispersion compensator 23 into an optimum value on the basis of outputs from the signal processing unit 25, a function of detecting an input break of an optical signal, and a function of stopping operations by the optical amplifier 22 and by the variable dispersion compensator 23 when it has detected an input break of an optical signal. When the control unit 26 cannot detect an optical signal higher than a prescribed level, it determines that an input break of an optical signal has occurred.
FIG. 3 illustrates the levels of optical signals, ASE (Amplified Spontaneous Emission) light, which is noise made in the optical amplifier 22, and an LOL (Light Of Loss) threshold value.
Conditions under which transponders can receive normal optical signals are determined by the power and the OSNR (Optical Signal-To-Noise Ratio), and LOL threshold values for detecting an input break of optical signals under those conditions are set.
In FIG. 3, an optical signal 1 of CH_n1 has sufficient optical power, and also clears the criterion of OSNR. An optical signal 2 of CH_n2 has optical power that is low but higher than the LOL threshold value, and that power has exceeded the minimum reception power. ASE light of CH_m is noise made in the optical amplifier 22, and its power is higher than the LOL threshold value in this case.
When the level of the ASE light is higher than the LOL threshold value, input breaks cannot be detected because there is ASE light even when an input break of optical signals has occurred.
A method in which the optimum points are searched for while monitoring the quality of an optical signal is used for adjusting the amount of dispersion compensation in the transponders 19a through 19n in the receiver side. When no optical signals are input to the transponders 19a through 19n, the quality of an optical signal cannot be monitored, and thus a method in which the optimum points are searched for cannot detect an input break, and this offers a risk that an inappropriate dispersion compensation amount might be set or that a process of searching for optimum points of dispersion compensation might be repeated endlessly because such optimum points cannot be found.
If an inappropriate dispersion compensation amount is set, the dispersion compensation amount has been shifted greatly when an optical signal is input again, and this causes a problem in which a process of searching for the optimum point of compensating for dispersion takes too long a time. Additionally, when a variable dispersion compensator continues its operation for searching for the optimum point with no optical signal having been input to it, the lifetime of that variable dispersion compensator is shortened.
Patent Document 1 discloses a configuration in which plural low-frequency signal sources are provided for generating low-frequency signals of different frequencies, optical signals of wavelengths λ1, λ2, . . . are amplitude modulated using a different low-frequency signal for each of the wavelengths of the light sources, and the amplitude-modulated optical signals are wavelength multiplexed so as to be output to an optical transmission line.
Patent Document 2 discloses a configuration in which a switch is provided for detecting a channel that is not transmitting optical signals so as to disconnect a line not transmitting optical signals when wavelength-division multiplexed optical signals are to be coupled.
Patent Document 3 discloses a configuration in which excitation light output levels of light sources are controlled so that the optical signal output level of an optical fiber amplifier can be constant and a wavelength-passing band of a wavelength-variable optical filter can be controlled in accordance with signals for controlling the light sources.
Patent Document 4 discloses a configuration in which the transmitter side modulates wavelength-multiplexed optical signals by using low-frequency signals, and the receiver side detects the low-frequency signals so as to control the passing band of a Fabry-Perot filter on the basis of results of the detection.
While the above patent documents disclose modulation of wavelength-division multiplexed optical signals using low-frequency signals, none of them discloses how to detect an input break of an optical signal when the level of ASE light is high.    Patent Document 1: Japanese Laid-open Patent Publication No. 2000-201106    Patent Document 2: Japanese Laid-open Patent Publication No. 2004-247780    Patent Document 3: Japanese Laid-open Patent Publication No. 11-317709    Patent Document 4: Japanese Laid-open Patent Publication No. 6-222237